The present invention is generally directed to crimped polymeric fibers and to nonwoven webs and laminates made from the fibers. More particularly, the present invention is directed to making crimped fibers more resilient to external forces, such as compressive forces, so that nonwoven webs made from the fibers retain their high loft characteristics.
High loft, low density webs and fabrics are used for a variety of technical applications. Filtration media utilize high loft fabrics where the density of the fabric (weight/unit volume) and the fiber sizes determine pore size, fiber surface area, and pressure drop through the fabric. These properties determine the functionality, efficiency and capacity of the filter.
These same properties of fiber size, density and loft or bulk affect fluid distribution and the capacity of high loft fabrics designed to hold, transport and distribute fluids in absorbent articles. For instance, such high loft and low density fabrics, particularly nonwoven webs, are used in such products such as towels, industrial wipers, incontinence products, infant care products such as baby diapers, absorbent feminine care products, professional health care articles, and loop materials for hook and loop fasteners, such as VELCRO fasteners.
Some of these products are often made with multiple layers of nonwoven fabrics to obtain a desired combination of properties such as softness, strength, uniformity, and liquid handling properties. For example, disposable baby diapers made from polymeric nonwoven fabrics may include a soft and porous liner layer which fits next to the baby""s skin, an impervious outer cover layer which is strong and soft, and one or more interior liquid handling layers which should be soft, absorbent, and have a very high loft.
Unfortunately, although methods exist for producing high loft and low density fabrics, the fabrics are typically subjected to a number of processes during conversion which compress the material and reduce the overall bulk that was created.
Compression can result from winding the fabrics during fabric manufacturing, winding of composites produced utilizing the high loft fabrics, fabrication steps where compression is necessary for adhesion, registration, etc., packaging where compression is used to minimize unit volumes for shipping and during use of the fabric. All of these steps reduce loft and bulk of the high loft fabrics. When the articles are unpacked for application or use or during use, the high loft webs need to recover their loft to the necessary fabric density to insure proper function. Fabrics which do not recover have poor performance and can result in product failures.
Filters will develop excessive pressure drop and will not perform as designed if the high loft structure does not recover. Absorbent articles, on the other hand, have reduced ability to hold and distribute fluids effectively which results in increased leakage.
Thus, a need currently exists for a process for producing high loft fabrics that are resilient to compressive forces. More specifically, high loft fabrics are typically made from crimped polymeric fibers. Thus, a need also exists for a process for producing crimped fibers that xe2x80x9cbounce backxe2x80x9d when compressed. A need further exists for a process for making webs that retain their high loft and low density characteristics.
The present invention recognizes and addresses the foregoing problems and others experienced in the prior art.
Accordingly, an object of the present invention is to provide improved crimped fibers and nonwoven fabrics made from the fibers.
Another object of the present invention is to provide a process for producing fibers having a resilient crimp.
A further object of the present invention is to provide a process for producing high loft nonwoven webs and laminates that are resilient to compressive forces.
Another object of the present invention is to provide a process for producing crimped polymeric fibers wherein a polymer and/or a monomer incorporated into the fibers is cross-linked after the fibers have been crimped.
Still another object of the present invention is to provide a process for producing resilient crimped fibers that contain a cross-linked polyethylene polymer.
These and other objects of the present invention are achieved by providing a process for producing resilient crimped fibers. In one embodiment, the process is directed to forming crimped fibers containing polyethylene. For example, the fiber can be a monocomponent fiber containing polyethylene or can be a multicomponent fiber having polyethylene as one of the components. For instance, in one embodiment, the crimped fiber can be a bicomponent fiber having a polyethylene component and a polypropylene component. Alternatively, the fiber can be made from a polymer blend containing polyethylene.
According to the present invention, once the fiber is crimped, the polyethylene contained within the fiber is cross-linked. It has been discovered by the present inventors that cross-linking the polyethylene makes the crimp contained within the fibers more permanent and more resilient to compressive forces. After cross-linking, the fibers exhibit a xe2x80x9cbounce backxe2x80x9d property in that they retake their original shape if compressed or otherwise compacted.
According to the present invention, there are various methods available in order to cross-link the polyethylene contained within the fiber. For instance, the polyethylene can be cross-linked by exposing the fiber to electron beam irradiation. In an alternative embodiment, a cross-linking agent can be combined with the polyethylene which initiates cross-linking during or after the fiber has been formed and crimped. For example, in one embodiment, the cross-linking agent can be a peroxide which causes polyethylene to cross-link when exposed to heat.
In an alternative embodiment, a silane can be used as a cross-linking agent. In particular, silane can be used as a cross linking agent when combined with a peroxide and a catalyst, such as a tin catalyst. Specifically, silane and a peroxide can be blended with a polymer, such as polyethylene, and can cause the polymer to cross link when the polymer is exposed to moisture.
In a further alternative embodiment, the cross-linking agent can be a photoinitiator, which initiates cross-linking of the polyethylene when subjected to electromagnetic radiation, such as ultraviolet radiation. Examples of photoinitiators include benzoins, benzoin ethers, benzophenones, acetophenones, thioxanones, aryladiazonium salts, and mixtures thereof.
Besides using a photoinitiator or in addition to using a photoinitiator, the polymeric fiber of the present invention can contain a monomer, such as a light reactive thermoset monomer. According to the present invention, the thermoset monomer can polymerize when exposed to light energy and provide rigidity to the fiber and/or can cause a polymer contained within the fiber to cross-link. For instance, in one embodiment, triallylcyanurate can be incorporated into a thermoplastic polymer, such as polyethylene, in an amount of at least about 0.25% by weight and particularly from about 0.25% to about 30% by weight.
Various crimped fibers may be used in the present invention including carded fibers, spunbond fibers, and meltblown fibers. The fibers can be crimped mechanically after fiber formation or naturally crimped during fiber formation. As used herein, a naturally crimped fiber is a fiber that is crimped by activating a latent crimp contained in the fiber.
Besides fibers, the present invention is also directed to nonwoven webs made from fibers having a resilient crimp. For instance, in one embodiment, the present invention is directed to making nonwoven webs out of crimped, bicomponent fibers. The fibers can be made according to a melt spinning process, such as a meltblown process or a spunbond process. In this embodiment, the process of the present invention includes the steps of meltspinning multicomponent fibers. The fibers contain a first polymeric component and a second polymeric component. According to the present invention, the first polymeric component may contain a cross-linking agent, such as a photoinitiator.
Once meltspun, the multicomponent fibers are crimped and formed into a nonwoven web. The nonwoven web is then cross-linked. One method is to expose the web to electromagnetic radiation, such as ultraviolet light, which activates a photoinitiator and/or a monomer for causing the first polymeric component to cross-link and thereby make the crimp contained within the fibers more resilient.
Besides using a photoinitiator, the first polymeric component can be cross-linked by using a peroxide or by using a silane and peroxide additive that cause cross-linking when exposed to a tin catalyst and water. In a further alternative embodiment, the first polymeric component can be cross-linked by being exposed to electron beam radiation.
In one embodiment, the first polymeric component is polyethylene, while the second polymeric component is polypropylene. The fibers can be made into continuous filaments.
Besides being directed to fibers and nonwoven webs, the present invention is also directed to laminates incorporating nonwoven webs made according to the present invention. For instance, a laminate can be constructed containing a first nonwoven web adhered to a second nonwoven web. The first nonwoven web can be a high loft, low density web containing cross-linked and crimped polymeric fibers made in accordance with the present invention.
For example, in one embodiment, a laminate can be constructed containing a nonwoven web made in accordance with the present invention attached to a spunbond web or a carded web made from, for instance, polypropylene fibers. This laminate can be used, for instance, as a liner and surge layer incorporated into a liquid absorbent article, such as a diaper. Alternatively, fabrics made in accordance with the present invention can be incorporated into multilayer filtration media.
Other objects, features and aspects of the present invention are discussed in greater detail below.